Blended Coatings from Polyaniline (PANI) and Chemically doped Sodium docecyl benzene sulphonate (SDBS)/SiO 2 Nanocomposites: Influence on Corrosion Protection of Mild Steel

In this work, investigation on corrosion protection performance of chemically synthesized polyaniline (PANI) / SiO 2 nanocomposite coatings has been carried out on Mild Steel (MS). Sodium docecyl benzene sulphonate (SDBS) doped conducting PANI / SiO 2 at different ratios was synthesized by employing in -situ polymerization technique. The developed coatings were characterized using Fourier transform infrared spectroscopy (FTIR), Thermo gravimetric analysis (TGA), Transmission electron microscopy (TEM), Contact angle, Atomic force microscopy (AFM) and corrosion analysis. The FTIR analysis indicates the strong interaction between PANI and SiO 2 nanoparticles. The contact angle study reveals the hydophillicity character of the nanocomposite coatings with a water contact angle of 74.9°. Corrosion resistanceof uncoated mild steel and the coated sample in 3.5 % NaCl aqueous solution has been evaluated using weight loss methods. Additionally the Electrochemical Impedance Spectroscopy (EIS) studies have been also conducted to evaluate the coorision protection characteristics of the coatings. PANI containing SiO 2 (PSC III) coating showed excellent resistance after immersion in 3.5 % NaCl solution for 1 month time period.


INTRODUCTION:
Corrosion process which involves gradual destruction of materials exposed to environmental conditions through chemical or electrochemical reactions has been a primary concern globally. Around 3% of the global gross domestic product (GDP) annually is effected by the corrosion process [1][2].Corrosion protection employing polymeric coatings have generated considerable research interests in the recent years.
have been reported to be widely employed as coatings over the metal substrates to these substrates against corrosion [3][4]. Amongst the various conducting polymers, polyaniline (PANI) based systems have been reported to demonstrate improved corrosion protection properties as compared with the other polymers [3].
PANI known for its conducting properties [5] has good stability, high electrical conductivity [6] in addition to energy storage and electrochemical properties. PANI has been employed for multitude of applications like sensors, super capacitors, electrochromic display, fuel cells rechargeable organic batteries, drug delivery as well as in corrosion protection coatings.
However, the major impediments of PANI includes limitations in processability predominantly due to its insolubility in majority of common solvents; brittle structure which prevents its wider applicability. These problems can be largely averted by changing the oxidation state of PANI, incorporation of dopants or preparing blends/composites of PANI with other polymers and reinforcing it with nanofillers [7].
Conducting polymer composites of PANI reinforced with nano fillers have been known to show its potentiality for corrosion protection. Encapsulated metal oxide based particles within the shell of PANI have bestowed improved physico-mechanical and chemical properties while retaining the conducting properties with synergism between the matrix polymer and the inorganic particles [8][9]. Several inorganic/organic particles like SiO2, TiO2, Graphene, Fe3O4, ZnO, ZnMoO4etc have been reinforced within PANI employing emulsion polymerisation technique to achieve desired attributes [10].
Among various inorganic particles, SiO2nanoparticles having a porous structure can be suitably modified for its potential application in corrosion protection coatings [11]. Al-Dulaimi A et al have reported single step insitu method of synthesising PANI/SiO2 composite having improved corrosion protection characteristics in corrosive solvents [12].
In the current investigation PANI/SiO2 nanocomposite coatings has been prepared using single step-in-situ-polymerisation technique. Sodium dodecyl benzene sulphonic acid (SDBS)was used as a dopant owing to its improved solubility. An in-depth study on the morphology of the synthesised nanocomposite coatings was carried out employing Fourier Transform Infrared spectroscopy (FTIR), Transmission electron microscopy, X-ray diffraction (XRD) and atomic force microscopy. The corrosion study was investigated through weigh loss method and the electrochemical impedance spectroscopy and contact angle study of the nanocomposite coatings have been reported.

Preparation of PANI/ SiO2composites coating on mild steel
Mild steel panels were coated at variable loadings of PANI/SiO2(PSC) and neat PANI formulated with epoxy resin to evaluate the corrosion resistance properties of the coatings.
Prior to coating the surface of the mild steel panels were subjected to gritting using emery paper followed by washing and cleaning with trichloroethylene and acetone to remove contamination if any on the surface. 6g of solid epoxy resin was dissolved in 4 ml xylene solution to which 1% PANI/SiO2 solution was added and ultrasonicated for 2 hrs. Then 1g hardener was added and the mixture was further sonicated for 1 hr to obtain the coating formulation. Xylene-butanol mixture at a ratio of 9:3 was added to maintain the viscosity of coating. Finally the coating formulation was coated onto mild steel substrate of 1 cm 2 usingdrop coating method and was dried at 600 ºC for 4 h. A coating thickness of 110± 10 µm was maintained in all the formulations.

Characterization of the PANI and its nanocomposite coatings
After the successful growth of polymer nanocomposite coatings, the microstructural andelectrochemical corrosion properties were investigated using various characterization tools. Thestructural information of polymer nanocompositescoatings was performed using Fourier transforminfrared (FTIR) spectroscopy (Thermo Scientific, Nicolet 6700, USA) in the attenuated totalreflectance mode from 400-4000 cm -1 . The X-ray diffraction pattern of the polymer nanocomposite coatings were examined using Shimadzu X-ray diffractometer with a measuringangle from 5 to 80º and the scan rate was maintained 5º/min. The surface

Electrochemical Impedance studies (EIS):
The electrochemical characteristics of as-prepared epoxy and PS coatings were tested using a electrochemical workstation in 3.5% NaCl solution. The exposed area of the coating is 1 cm 2 . The EIS and Tafel are measured in a three-electrode system. The coated substrate was a working electrode, saturated calomel electrode (SCE) was taken as reference electrode and platinum as a counter electrode. The following equations was used to evaluate the protection efficiency of the coating.
Where, P(%) is the protection efficiency, Rct and Rct* is the charge transfer resistance of coated and uncoated substrate, icorr and icorr* is the corrosion current density coated and uncoated substrate.

Weight loss method
To further evaluate corrosion properties of as-prepared PANI/SiO2 coated mild steel was extensively studied by weight loss method. The bare and PANI/SiO2 coated mild steel with a dimension of 2cm×3cm were immersed in 1.0 mol L -1 NaCl medium for 30 days.
Before and after immersion, the bare and coated mild steel were weighed in the electronic balance. After 30 days, the test specimens were removed from the solution and washed exhaustively withdistilledwater, followed by acetone and dried in air. The changes in the coated substrate were visually inspected after 30 days of immersion. Moreover, the changes in weight were calculated according to the weight loss (g cm -2 h -1 ) formula as follows The corrosion rate (CR) of the PS coated mild steel substrate was estimated by weight loss method using the following equation.
Where, W1 is the initial weight (before immersion) of the sample (mg), W2 final weight (after immersion) of the sample (mg), a is the surface area of the sample (cm 2 ), t is the end time of immersion (h) and d is the density of bare mild steel substrate (7.85 g cm -3 ). Similarly, thebending vibration of Si-O-Si peak at 1056 cm-1 was found to be shifted to lower wavenumbers. The behaviours indicated interaction between PANI and SiO 2 nanoparticles.

3.2XRD analysis
The XRD pattern of PANI and PANI/ SiO2is representedin Fig 2. The peak related to PANI was observed at 2θ = 20 and 25° , indicating (020) (200) semi-crystalline planes as reported by Chin et al. [22]. However, with the incorporation of SDBS, the intensity of the d200 peak reduced in presence of dopant. Further as observed from the x-ray diffractograms of PANI/SiO2 nanocomposite it is observed that a relatively intense peak at 2θ = 20was observed which revealed higher crystalline nature of PANI in presence of the dopant and SiO2.
On contrary, Chei et al. [22], had reported in their work that SiO2 has no influence on the crystallinity of PANI. Hence, it can be concluded that addition of SiO2 has considerable effect on PANI and that might due to the absorption of PANI on SiO2 surface in presence of SDBS [23].

Contact angle
This wettability characteristics of the coated surfaces was testedusing contact angle measurements. The data of water contact angle measurements showed that the surface paint had shown fig 5. The hydrophilic nature because the contact angle is less than 90 degree.The measurements revealed that the surface contact angle of PANI Neat and PANI/SiO2 -III nanoomposite was found to be 64° and 73°, respectively asshown in Fig.5. This shows an improvement of thePANI/SiO2 in the wettability characteristics as compared to the neat counterpart. Therefore the results indicate that the incorporation of SiO2 reduces the hydrophillicity.   However, the PANI/SiO2 coated panels showed improved performance in which the weight loss was 4.98%. Comparing the CR rates, it is evident from the test results reported in Table 1 , after 30 days of immersion of the blank mild steel , the CR value was 1.59/year which reduced drastically to 0.61 mm/year approximately to the tune of 160% thus revealing the efficacy of the coated nanocomposite panels [21].

Electrochemical studies of PS/MS Coatings
The corrosion properties of as-prepared coatings were analyzed by EIS and Tafel